Optimal subpixel arrangement for displays with more than three primary colors

ABSTRACT

A display comprises a plurality of first, second, third and fourth display elements, which are controllable to display first, second, third and fourth primary colors, respectively. A particular pair of said display elements, which represents a maximum color distance compared to other pairs of said display elements, is arranged such that there is a minimal spatial distance between the elements of said particular pair or that the elements of said particular pair are situated next to each other.

The present invention relates to a display comprising a plurality of first, second, third and fourth display elements, which are controllable to display first, second, third and fourth primary colors, respectively.

Vision is the sense, mediated by the eyes, by which the qualities of an object (such as color, luminosity, shape and size) constituting its appearance are perceived.

Color is defined as an attribute of visual perception consisting of any combination of chromatic and achromatic content. This attribute can be described by chromatic color names such as yellow, orange, brown, red, pink, green, blue, purple, etc., or by achromatic color names such as white, grey, black, etc., and qualified by bright, dim, light, dark, etc., or by combinations of such names.

A perceived color depends on the spectral distribution of the color stimulus, on the size, shape, structure and surround of the stimulus area, on the state of adaptation of the observer's visual system, and on the observer's experience of the prevailing and similar situations of observations.

The unrelated attributes of color are brightness, hue and saturation. Brightness is the attribute of a visual sensation according to which an area appears to emit more or less light. Hue is an attribute of a visual sensation according to which an area appears to be similar to one of the perceived colors, e.g. red, yellow, green, and blue, or to a combination of them. Saturation is the colorfulness, chromaticness, of an area judged in proportion to its brightness.

The related attributes of color are lightness, colorfulness and chroma. Lightness is defined as the brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting. Colorfulness is an attribute of a visual sensation according to which the perceived color of an area appears to be more or less chromatic. Chroma is defined as the colorfulness, chromaticness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting.

In the retina of the eye there are three different types of light sensors. These sensors are called the L, M and S cones, which are sensitive to light with long (L), medium (M) and short (S) wavelengths, respectively. Each type of sensor is connected with neurones to the brain. When light falls on a cone it will start to send pulses to the brain when it is sensitive to the wavelength of the light. FIG. 1 shows the spectral sensitivities of L, M and S cones in the human eye. The more light falls on the cones the quicker they will send pulses (“fire spikes”) to the brain.

The color of the light that enters the eye is determined by the relative amount of pulses that each of the three types of cones sends to the brain. Blue light (wavelength approximately 400-450 nm), for example, results in more spikes from the S cones than from the L cones or the M cones.

Because the human eye has only three types of cones, there are a number of different light spectra that give the same color sensation. For example, sunlight and the light from a fluorescent lamp are both perceived as white in color, but whereas the sunlight has a very broad spectrum with about equal intensity for each wavelength, the fluorescent lamp has a spectrum with only a few peaks. This effect of different light spectra giving the same color sensation is called metamerism, and two spectra which give the same color sensation are called metamers.

Another effect of having only three types of cones is that different colors can be made by adding together the light of two light sources while varying the relative intensity of these light sources. If red light and green light are mixed, it may be perceived as yellow. If a first light source emitting red light is set to full intensity and a second light source emitting green light is set to zero intensity, and the intensity of the green light is increased while the intensity of the red light is decreased, the color changes from red, to orange, to yellow, and finally to green.

Displays use this principle to make many colors with only three primary colors; usually red, green and blue.

In order to predict the color sensation that we get from the light that enters our eyes, a number of models have been developed. One of these models, which is most commonly known and which is standardised by the CIE (Commission Internationale d'Éclairage—International Commission on Illumination) is the CIE 1931 model. It defines three spectral matching functions for the standard observer that can be used to calculate the tri-stimulus values X, Y, and Z, respectively, for a light with a certain spectrum. From these tri-stimulus values the chromaticity coordinates x and y can be calculated as follows: $\begin{matrix} {x = \frac{X}{X + Y + Z}} & (1) \\ {y = \frac{Y}{X + Y + Z}} & (2) \end{matrix}$

Y is related to the perceptual attribute brightness, the x and y coordinates determine the chromaticity, where x is the red-green axis and y is the yellow-blue axis.

The relation between colors (while ignoring the intensity, Y) can now be plotted in a two-dimensional chromaticity diagram, such as FIG. 2. It shows the chromaticity coordinates of the spectral colors by the curved line and indicates the corresponding wavelengths in nanometres (nm). Chromaticity coordinates for all visible colors are on the horseshoe shaped area inside the curved line. The straight line at the bottom of the chart (the purple line) connects the red and the blue spectral colors, so that non-spectral colors obtained by mixing red and blue (e.g. purple, violet, etc.) are located along this line. The chromaticity coordinate of a white object in daylight is designated D in FIG. 2. The direction and the distance of a certain point in the chromaticity diagram to the white point determine its hue and saturation.

As mentioned previously, mixing the light of two colors can create a new color. The chromaticity coordinate of this new color is on an imaginary straight line between the two colors. Mixing green (G) and cyan (C) will for instance give a color whose chromaticity coordinate is on the broken line 21 between G and C as given in FIG. 2. By adding a third color, e.g. red (R), all colors within an imaginary triangle, spanned by R, G, and C can be made. By mixing light of six different primary colors (e.g. R, Y, G, C, B, M), all colors with chromaticity coordinates in the patch R, Y, G, C, B, M, i.e. inside a polygon, the corners of which are R, Y, G, C, B, and M, can be made.

The chromaticity diagram only shows the proportions of tristimulus values; hence bright and dim colors having the same tristimulus proportions belong to the same point. For this reason, the illuminant point D also represents grey colors; and orange and brown colors, for example, tend to plot at similar positions to each other.

The subject matter of color vision is further elucidated in e.g. Roy S. Berns, Fred W. Billmeyer, and Max Saltzman; Billmeyer and Saltzman's Principles of Color Technology, 3rd Edition; ISBN 0-471-19459-X, hereby incorporated in its entirety by this reference.

The present invention relates to the field of displays in general, and in particular to liquid crystal displays (LCD), cathode ray tube (CRT) displays, flat intelligent tube (FIT) displays, light emitting diode (LED) displays, all of which will be explained briefly in the following, as well as to plasma display panels (PDP), PolyLED displays, organic light emitting displays (OLED), field emission displays (FED), and foil displays.

In prior art, liquid crystal displays have proven themselves suitable for various applications which necessitate compactness and low power consumption. A liquid crystal display (LCD) is a flat panel display device having the advantages of small bulk, small thickness and low power consumption.

LCDs have been used in connection with portable devices such as mobile telephones, portable computers, electronic calendars, electronic books, televisions or video game controls and various other office automation equipment and audio/video machinery, etc.

LCDs control an electric field which is applied to a liquid crystal material having a dielectric anisotropy to modulate light, thereby displaying a picture or an image, all in a fashion that is known per se as is recognized by those skilled in the art. Unlike display devices that generate light internally—such as electro luminescence (EL) devices, cathode ray tubes (CRT) and light emitting diodes (LED)—LCDs use an external light source.

Normally, an LCD display is designed as a liquid crystal panel, comprising a matrix of essentially rectangular display elements (pixels) which are controllable to transmit or reflect light depending on the properties of the liquid crystal mixture, which generally is injected between two transparent substrates, the display in addition comprising row and column conductors for supplying voltages to selected parts of the display, via associated electronics such as row and column drivers, as will be recognized by the skilled man.

LCD devices are broadly classified into transmissive type devices and reflective type devices, depending on the method of utilizing light. Transmissive type LCDs include a back light unit for supplying light to the liquid crystal panel.

Light emitting diodes (LED) have been used to create big screen devices such as jumbo-TVs. Depending on the desired pixel size, a number of red, green and blue light emitting diodes may be grouped together to form a single display element, corresponding to a pixel in an LCD display. Such display elements are subsequently arranged in a rectangular matrix and connected to necessary electronics as will be recognized by the skilled man.

FIG. 3 is a schematic illustration of the fundamental principle of the cathode ray tube (CRT), which is comprised in many TVs in use today as well as in many other display devices. A cathode 31, for instance a heated filament, is arranged inside a glass tube 32, in which a vacuum has been created. Electrons are naturally released from the heated cathode 31 and migrate into the tube 32. An anode 33 attracts the electrons, which are released from the cathode 31, thus forming a beam or ray of electrons 34. In the cathode ray tube 32 of a television set, the beam of electrons 34 is focused by a focusing anode 33 into a tight beam and then accelerated by an accelerating anode 35. The beam of electrons 34 flies through the vacuum inside the tube 32 and hits a flat screen 36 at the other end of the tube 32. This screen 36 is coated with phosphor 37, which glows when struck by the electron beam 34. A conductive coating inside the tube soaks up the electrons which pile up at the screen-end of the tube.

In order to provide means to guide the beam 34, the tube 32 in a typical CRT display device is wrapped in steering coils 38, 39. The steering coils 38, 39 are simply copper windings, which are able to create magnetic fields inside the tube, and the electron beam 34 responds to the fields. A first set of coils 38 creates a magnetic field that moves the electron beam vertically, while a second set of coils 39 moves the beam horizontally. By controlling the voltages applied to the coils 38, 39, the electron beam 34, can be positioned at any point on the screen 36.

A color CRT display comprises three electron beams, typically denoted as the red, green and blue beams, which move simultaneously across the screen. Instead of the single sheet of phosphor which is arranged at the screen in black-and-white CRT display devices, the screen in a color CRT display is coated with red, green and blue phosphors arranged in dots or stripes. On the inside of the tube, very close to the phosphor coating, there is arranged a thin metal screen, i.e. the shadow mask. This mask is perforated with very small holes that are aligned with the phosphor dots (or stripes) on the screen.

A red dot may be created by firing the red beam at the red phosphor, whereas green and blue dots are created in a corresponding fashion. To create a white dot, red, green and blue beams are fired simultaneously—the three colors mix together to create white. To create a black dot, all three beams are turned off as they scan past the dot. All other colors on a color CRT display are combinations of red, green and blue. CRT displays are typically time sequential displays, which implies that an image is built up by repeatedly scanning the beam(s) over the screen, whereupon an image is displayed, all in a manner known per se as will be appreciated by the skilled man.

The Flat Intelligent Tube (sometimes referred to as FIT or FIT) is a new cathode ray tube (CRT) technology without a shadow mask. The primary function of the shadow mask, i.e. color selection, is managed by an electronic control system that guides the electron beams over the correct phosphor lines. The position of the beams is detected by means of dedicated structures on the faceplate.

FIG. 4 is a simplified representation of the tracking principle in a FIT. In the FIT, the beams 34 are scanned along horizontal phosphor lines 41, in contrast to mask-less CRTs of the index type developed in the past in which a single beam was scanned perpendicularly to the vertical phosphor lines. The FIT approach is quite similar to that of a CD-player wherein a laser beam is guided over a spiral by means of a tracking system. The beam 34 is scanned along a horizontal phosphor line 41 and any deviation from this line is corrected by means of a feedback system. On tracks situated above and below each phosphor line 41, position detectors 42 are present (e.g. conducting stripes that measure the current). A control system 43, fed by information from these detectors 42, drives correction coil(s) 44 in such a way that the beam trajectories coincide with the phosphor lines 41.

In the CRT and FIT displays, the phosphor dots or stripes constitute the display elements, which accordingly are controllable to emit light having a predetermined wavelength (color).

In prior art RGB color displays, the displayable color gamut is limited to a color triangle, which is spanned by three primary colors, e.g. red, green and blue (as illustrated in FIG. 2). Colors outside this color triangle, e.g. gold and turquoise (in a case where the primary colors are red, green and blue), cannot be displayed and are consequently clipped towards colors that can be displayed, e.g. more unsaturated yellow and more bluish green. It is known that adding one or more additional primary colors to the three primary colors used in most present applications offers a possibility to expand the displayable color gamut.

Spatial resolution is the ability of a display system to display two objects that are close to each other as separate dots. For all display types that cannot project various color pixels on top of each other, the addition of a sub-pixel with a different-colored primary yields a reduction in the spatial resolution of the display if the number of sub-pixels remains equal.

The smallest switching element is the sub-pixel. If the sub-pixels are made smaller, there can be four sub-pixels in one pixel having the same size as a pixel with three sub-pixels. This however is costly and generally speaking resolution decreases as the amount of sub-pixels increases. If, on the other hand, the size of sub-pixels is kept constant and four, instead of three, sub-pixels are used to form a pixel, the pixel resolution will decrease.

Furthermore, the addition of more than three colors may result in errors relating to color, luminance and image homogeneity.

It is accordingly a disadvantage that the addition of one or more primary colors in matrix displays results in a reduction of the overall image quality.

It is an object of the invention to provide a display, wherein the reduction of the overall image quality, particularly the color and luminance errors, resulting from the addition of one or more primary colors, is limited.

It is another object of the invention to provide a display, wherein an optimal arrangement of the display elements results in an increased homogeneity in color and luminance.

It is another object of the invention to provide a display, wherein black and white transitions on sub-pixel level without color artefacts can be generated using sub-pixel algorithms.

The present invention relates to a display comprising a plurality of first, second, third and fourth display elements, which are controllable to display first, second, third and fourth primary colors, respectively, characterized in that a particular pair of said display elements, which represents a maximum color distance compared to other pairs of said display elements, is arranged such that there is a minimal spatial distance between the elements of said particular pair or that the elements of said particular pair are situated next to each other.

The measures as defined in claims 2-6 have the advantages that they constitute increasingly optimal solutions to the problem of maximizing color distance while minimizing the spatial distance between display elements.

The measures as defined in claims 7-12 have the advantage that these colors are particularly suitable for use in color displays.

The measure as defined in claim 13 has the advantage of increased image quality due to an improved distribution of the luminance.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

Essentially speaking, the invention relates to a new and innovative method of arranging the display elements of a matrix display having four or more primary colors. According to the invention, the optimal arrangement will result in the best homogeneity in color and luminance. In addition, the arrangement according to the invention will limit the color and luminance errors.

An important aspect of the invention is the fact that although the display resolution on pixel level is lower, black and white transitions on sub-pixel level can be made without color artefacts as illustrated in FIG. 14 and FIG. 15 when using so-called sub-pixel algorithms.

FIG. 1 shows the spectral sensitivities of L, M and S cones in the human eye.

FIG. 2 is a chromaticity diagram.

FIG. 3 is a schematic illustration of the fundamental principle of a cathode ray tube (CRT).

FIG. 4 is a simplified representation of the tracking principle in a Flat Intelligent Tube (FIT).

FIG. 5 is a schematic illustration of the stripe-arrangement of sub-pixels in prior art color displays.

FIG. 6 is a schematic illustration of the mosaic-arrangement of sub-pixels in prior art color displays.

FIG. 7 is a schematic illustration of the delta-arrangement of sub-pixels in prior art color displays.

FIG. 8 is a schematic illustration of an RGBYMC-stripe-arrangement of sub-pixels in a six color display.

FIG. 9 is a schematic illustration of the arrangement of sub-pixels in a six-color display, wherein RGB-stripes and YMC-stripes alternate in subsequent rows.

FIG. 10 is a schematic illustration of the arrangement of sub-pixels in a six-color display, wherein RGB-stripes and YMC-stripes alternate in subsequent rows and columns.

FIG. 11 is a schematic illustration of the arrangement of sub-pixels in a six-color display according to a first embodiment of the invention, wherein RGB-stripes and CMY-stripes alternate in subsequent rows.

FIG. 12 is a schematic illustration of the arrangement of sub-pixels in a six-color display according to a second embodiment of the invention, wherein RGB-stripes and CMY-stripes alternate in subsequent rows and columns.

FIG. 13 is a schematic illustration of the arrangement of sub-pixels in a six-color display according to a third embodiment of the invention, wherein RGBCMY-stripes and CMYRGB-stripes are arranged alternatingly.

FIG. 14 is a schematic illustration of the smallest possible white display element in a prior art RGB display.

FIG. 15 is a schematic illustration of the smallest possible white display element in a display with an RGB/CMY sub-pixel arrangement according to the first embodiment of the invention.

Prior art multicolor displays comprise displays with red, green and blue primary colors; and an additional primary color such as yellow or white. In an LCD with four primary colors, each pixel is built up out of four “sub-pixels”: e.g. a red, a green, a blue and a yellow sub-pixel constitute a pixel.

When selecting an additional primary color, its impact on the luminance and the color gamut of a display should be taken into account. When considering the luminance alone, a primary color with a high luminance, such as those in the triangle yellow-white-green, appears desirable. Regarding the color gamut, with a view to extending the color gamut as much as possible, a highly saturated yellow, cyan or magenta would be preferred.

Yellow is furthermore a color which carries much brightness, and therefore its absence is easily detected; and this is why adding more saturated yellow colors generally is most appreciated from a perception point of view. Considering all requirements, a yellow primary would be the best choice of an additional primary color in an RGB-display.

FIG. 1 illustrates the sensitivity of the cones in the human eye to light of various colors. The eye is very sensitive to yellow light (570 to 580 nm), which is why adding a yellow primary color to a prior art display with only red, green, and blue primary colors (RGB-display) would substantially improve the overall brightness of a displayed image and the image quality.

A color other than yellow could nevertheless be a suitable fourth primary color if images of some special type were to be displayed. There may be several applications relating to the field of medical imaging or to the field of printing, wherein the first choice of an additional primary color would be a color other than yellow. Although the colors red, blue, green, cyan, magenta and yellow are mentioned as suitable colors in preferred embodiments of the invention, this should not be considered as a limitation on the invention.

In current color displays (with primary colors red (R), green (G) and blue (B)), the sub-pixels are usually configured according to one of the three patterns shown in FIGS. 5, 6, and 7.

FIG. 5 is a schematic illustration of the stripe-arrangement of sub-pixels in prior art color displays. The stripe-arrangement implies a simple array design, simple fabrication procedures and simple driving circuitry, but a poor color homogeneity.

FIG. 6 is a schematic illustration of the mosaic-arrangement of sub-pixels in prior art color displays. The mosaic-arrangement implies a simple array design, and a better color homogeneity, but at the cost of a more difficult fabrication procedure, and a more complex driving circuitry.

FIG. 7 is a schematic illustration of the delta-arrangement of sub-pixels in prior art color displays. The delta-arrangement implies the best color homogeneity and a simple driving circuitry, but at the cost of a more difficult fabrication procedure and a more complex array design.

The stripe arrangement of FIG. 5 is the most popular, followed by the mosaic arrangement of FIG. 6 and the delta arrangement of FIG. 7. The embodiments of the invention will mainly be described with reference to the stripe arrangement, which should not be considered to be a limitation on the invention, since the invention can be embodied in a variety of display arrangements and display types.

In human perception, homogeneity has an impact on the overall perceived quality of a display. The problems relating to deficiencies in the perceived homogeneity of a display which the invention seeks to remedy are most readily illustrated in the case of a display with six colors, namely red, green, blue, yellow, magenta and cyan (an RGBYMC-display). FIG. 2 illustrates exemplary chromaticity coordinates of R, G, B, Y, M and C in a RGBYMC display. FIG. 8 is a schematic illustration of an RGBYMC-stripe-arrangement of sub-pixels in a six-color display. The display screen comprises a matrix of pixels, which in turn are built up of a repeated arrangement of red (R), green (G), blue (B), yellow (Y), magenta (M) and cyan (C) sub-pixels.

The display is furthermore comprised of several components such as row and column conductors (not shown), connected to electronics (not shown), such as row and column drivers, all in a manner known by the skilled man and therefore not described here in order not to obscure the invention in unnecessary detail. If a six-color display is designed in a prior art stripe-arrangement according to FIG. 8, the color homogeneity (color mix) will be very poor.

A much better color homogeneity can be obtained with an alternating stripe arrangement of RGB- and YMC-stripes according to FIG. 9, wherein RGB-stripes and YMC-stripes alternate in subsequent rows.

An even better embodiment is to alternate RGB-stripes and YMC-stripes between subsequent rows and columns according to FIG. 10, which nevertheless results in approximately the same homogeneity in color as the previous arrangement.

The distance between two colors, the color distance, denotes the distance between two colors in the CIE chromaticity diagram, i.e. the length of a straight line, drawn through the points (chromaticity coordinates) which represent said two colors. In FIG. 2, the color distance between green and magenta is illustrated with a broken straight line going from green (G) to magenta (M).

If a first color has the chromaticity coordinates x₁, y₁ in the CIE chromaticity diagram and a second color has the chromaticity coordinates x₂, y₂ in the CIE chromaticity diagram, one way of calculating the color distance D_(C) is according to equation (3). D _(C)=√{square root over ((x ₁ −x ₂)²+(y ₁ −y ₂)²)}  (3)

The color distance may nevertheless also be calculated in another way, wherein the human perception of the colors is taken into account, and wherein the color distance is the perceived color distance, either by individuals or groups of individuals, or in some other way.

The color distance could for instance be calculated in the CIE 1976 Yu′v′ color space, wherein u′ and v′ are calculated from X, Y and Z according to the equations (4) and (5). $\begin{matrix} {u^{\prime} = \frac{4\quad X}{\left( {X + {15\quad Y} + {3\quad Z}} \right)}} & (4) \\ {v^{\prime} = \frac{9\quad Y}{\left( {X + {15\quad Y} + {3\quad Z}} \right)}} & (5) \end{matrix}$

The color distance between two colors may then be determined according to equation (6). D _(C)=√{square root over ((u ₁ ′−u ₂′)²+(v ₁ ′−v ₂′)²)}  (6)

According to the invention, the best homogeneity can be obtained by arranging those primary colors that have the maximum color distance (i.e. are most distant from each other in the color space) at a minimal spatial distance from each other (i.e. close to one another), preferably next to each other, and furthermore by arranging the display elements so that the sub-pixels of the same colors are equally far apart spatially both in the horizontal direction and the vertical direction. Such a solution would link red to cyan, green to magenta and blue to yellow.

In FIG. 2, the maximum color distance between green (G) and any other of the five primary colors, namely the distance between green (G) and magenta (M) is illustrated as the broken line 21. Analogously, the maximum color distance between blue (B) and any other of the primary colors, namely the distance between blue (B) and yellow (Y), is illustrated as the broken line 22; and the maximum color distance between red (R) and any other of the primary colors, namely the distance between red (R) and cyan (C), is illustrated by the broken line 23.

According to a first embodiment of the invention, the display elements are arranged in an alternating stripe configuration, which would yield RGB- and CMY-stripes according to FIG. 11.

In the magnified section of FIG. 11, the spatial distances between the red (R) and cyan (C) pixel (111), the spatial distance between the green pixel (G) and the magenta (M) pixel (112), and the spatial distance between the blue pixel (B) and the yellow (Y) pixel (113) are illustrated.

In the embodiment of FIG. 11, said spatial distances 111, 112 and 113 are ideally minimized by arranging the display elements next to each other. The elements may be separated by a small physical distance between the elements, as illustrated in the magnified section of FIG. 11. According to a second embodiment of the invention, an alternative configuration of RGB- and CMY-stripes, illustrated in FIG. 12, is almost as good as the previous one.

According to a third embodiment, an alternative arrangement of RGBCMY- and CMYRGB-stripes is proposed according to FIG. 13. Since the sub-pixels with the same color are not distributed equally in the horizontal and vertical directions, this embodiment is not a preferred one.

Using sub-pixel algorithms, the perceived resolution of a display can be increased substantially. This is already known for conventional RGB displays, but similar algorithms may be developed for RGBYMC displays. Such algorithms will perform optimally when white can be created at “near sub-pixel level”. Of all possible arrangements, the arrangement of FIG. 11 again results in the best performance regarding this aspect, since white can be created with three pixels in the horizontal direction (RGB) or two pixels in the vertical direction (RC, GM, BY) starting from any sub-pixel in the image. This is in fact better than in RGB mosaic displays where three pixels in both the horizontal and the vertical directions are required. Note that the idea is equally true when the sub-pixel configuration for rows and columns is exchanged.

FIG. 14 is a schematic illustration of the smallest possible white display element in a prior art RGB display. In the display of FIG. 14, 4 rows comprising 5 pixels each, making a total of 20 pixels, are illustrated. Every pixel has red, green and blue sub-pixels, resulting in a total of 60 illustrated sub-pixels.

FIG. 15 is a schematic illustration of the smallest possible white display element in a display with an RGB/CMY sub-pixel arrangement according to an aspect of the invention. In the display of FIG. 15, 3 rows comprising 3 pixels each, making a total of 9 pixels, are illustrated. Every pixel has red, green, blue, cyan, magenta and yellow sub-pixels, arranged as in the display of FIG. 11, resulting in a total of 54 illustrated sub-pixels.

In the conventional RGB display of FIG. 14, white light can be produced by activating a pixel comprising red, green and blue sub-pixels, requiring a total of three sub-pixels in the horizontal direction to be activated in order to produce white light as indicated by the arrow. Using sub-pixel algorithms, white light can be produced by the display of FIG. 15 by either activating the red sub-pixel and the cyan sub-pixel, the green sub-pixel and the magenta sub-pixel, or the blue sub-pixel and the yellow sub-pixel, as indicated by the arrows. White light can also be produced by activating the red, the green and the blue sub-pixels of a pixel or by activating the cyan, the yellow and the magenta sub-pixels of a pixel.

Although there are three times as many pixels in the RGB display of FIG. 14 as in the RGB/CMY display of FIG. 15, the RGB/CMY display of FIG. 15 can enable more than 1.5 times the resolution of the RGB display for black and white images (such as text).

With a view to providing an even better image quality, the arrangement of display elements which are to be arranged next to each other could comprise arranging the display elements with the highest luminance signals at the greatest distance from each other. The principle will be elucidated in the following, with reference to an exemplary RGBY display. This particular display type should however not in any way be considered as a limitation on the invention.

The exemplary RGBY display is a regular matrix arrangement comprising rows and columns of identical pixels, each pixel comprising 2×2 sub-pixels (i.e. each pixel comprises four different sub-pixels, one of every color, arranged in 2 rows with 2 sub-pixels in every row).

In any RGBY matrix display, the pair of green display elements and yellow display elements typically represent a higher luminance than any other pair of display elements such as red display elements and blue display elements.

By arranging the green display elements and the yellow display elements on the diagonals of the pixels, instead of on the same row (or column), the spatial distance between the particular pair of display elements which represent the highest luminance is maximized.

This can for instance be achieved by arranging the red display elements and the yellow display elements on a first row, and the green display elements and the blue display elements on a second row, instead of arranging the red display elements and the blue display elements on a first row and the green display elements and the yellow display elements on a second row. In this way, the distance between the centres of the yellow and green display elements would be equal to or larger than any other spatial distance between any other pair of display elements, namely approximately √{square root over (2)}=1.41 times the distance between the green display elements and the blue display elements for instance.

Hence a new and innovative display which presents the best homogeneity in color and luminance and limits the color and luminance errors and maximizes the resolution for images comprising black and white text has been proposed.

The illustrated arrangements of the pixels in the displays should not be considered to constitute a limitation, since pixels and sub-pixels may be of various regular or irregular shapes and arranged in a variety of regular or irregular patterns.

The display according to the present invention may, for example, be realized as a separate, stand-alone unit, or may alternatively be included in, or combined with, a mobile terminal for a telecommunications network, such as GSM, UMTS, GPS, GPRS or D-AMPS, or another portable device of an existing type, such as a Personal Digital Assistant (PDA), palmtop computer, portable computer, electronic calendar, electronic book, television set or video game control, as well as various other office automation equipment and audio/video machinery, etc.

The invention has mainly been described with reference to main embodiments. However, embodiments other than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims. All terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the [element, means, component, member, unit, step etc.]” are to be interpreted openly as referring to at least one instance of said element, means, component, member, unit, step etc. The steps of the methods described herein do not have to be performed in the exact order disclosed, unless explicitly specified. 

1. A display comprising a plurality of first, second, third and fourth display elements, which are controllable to display first, second, third and fourth primary colors (G, B, M, Y), respectively, characterized in that a particular pair (G, M; B, Y) of said display elements, which represents a maximum color distance compared to other pairs of said display elements, is arranged such that there is a minimal spatial distance between the elements of said particular pair (G, M; B, Y) or that the elements of said particular pair are situated next to each other.
 2. A display according to claim 1, wherein the two particular pairs of display elements (G, B; M, Y), which represent the greatest color distances compared to other pairs of said display elements, are arranged with minimal spatial distances between the elements of said particular pairs, or such that the elements of said particular pairs are arranged next to each other.
 3. A display according to claim 1, further comprising fifth display elements, which are controllable to display a fifth primary color.
 4. A display according to claim 3, wherein the three particular pairs of display elements, which represent the greatest color distances compared to other pairs of said display elements, are arranged with minimal spatial distances between the elements of said particular pairs, or such that the elements of said particular pairs are arranged next to each other.
 5. A display according to claim 3, further comprising sixth display elements, which are controllable to display a sixth primary color.
 6. A display according to claim 1, wherein the display is a matrix display.
 7. A display according to claim 1, wherein one of the primary colors is one of the colors red, green, blue, yellow, cyan or magenta.
 8. A display according to claim 1, wherein two of the primary colors are two of the colors red, green, blue, yellow, cyan and magenta.
 9. A display according to claim 1, wherein three of the primary colors are three of the colors red, green, blue, yellow, cyan and magenta.
 10. A display according to claim 1, wherein four of the primary colors are four of the colors red, green, blue, yellow, cyan and magenta.
 11. A display according to claim 3, wherein five of the primary colors are five of the colors red, green, blue, yellow, cyan and magenta.
 12. A display according to claim 5, wherein the primary colors are red, green, blue, yellow, cyan and magenta.
 13. A display according to claim 1, wherein a first display element and a second display element both are to be arranged next to a third display element, and wherein the spatial distance between a particular pair of said first display element, said second display element and said third display element, which represent the greatest luminance, is equal to or larger than any spatial distance between any other pair of said first display elements, second display elements and third display elements, respectively.
 14. A display according to claim 1, wherein said display is a liquid crystal display (LCD), a cathode ray tube (CRT) display, a flat intelligent tube (FIT) display, a light emitting diode (LED) display, a plasma display panel (PDP), a PolyLED display, an organic light emitting display (OLED), a field emission display (FED), or a foil display. 